The Census was held 7 April 1861. The closest sighting of a possible Naiad for Samuel to this date I can find is
The Times, 12 April, 1861
Ship News
Dover, April 11
Passed down channel, 4pm - The Blanche - The Naiad, of Whitby, N R B Q - The Adonis, of Waterford - L M D R
Does N R B Q refer to "alphabetic single hoist flags"?